Current applications for fine paper require particular attention to sizing before conversion or end-use, such as high speed photocopies, envelopes, forms bond, including computer paper, and adding machine paper. Paper is conventionally sized by addition of sizing agents to the "wet end" of the paper process (internal addition), i.e., to the pulp before sheet formation, or by addition of sizing agents to the surface of already formed paper sheet that has been at least partially dried (surface sizing).
Alkyl ketene dimers (AKD's) and alkenylsuccinic anhydrides are widely used paper sizing agents. Although they are described in the literature as being useful for both internal and surface sizing, they are generally not used for surface sizing commercially. Cellulose reactive sizes, such as ketene dimers and alkenylsuccinic anhydrides display high sizing efficiency, but may cause problems in size reversion, toner adhesion and high speed paper converting. Variable coefficient of friction is at least one factor leading to the problems in high speed converting operations.
Recently alkenyl ketene dimers and ketene multimers have been described that are useful for internal and surface sizing and that overcome the deficiencies in high speed converting. These materials are disclosed in U.S. Pat. No. 5,846,663, which is incorporated herein by reference in its entirety, and in European Patent Application No. 0,666,368 A3, which corresponds to commonly owned U.S. Pat. No. 5,685,815, which is incorporated herein by reference in its entirety. Precis.RTM.2000 and Precis.RTM.3000 sizing agents (available from Hercules Incorporated, Wilmington, Del.) are examples of such sizes. They are widely used commercially for internal sizing, but not for surface sizing because they do not contribute to good toner adhesion and other surface properties.
Cellulose non-reactive sizes have been used for some time as surface sizes. Examples of such materials are starch and other polymeric sizes such as copolymers of styrene with vinyl monomers such as maleic anhydride, acrylic acid and its alkyl esters, acrylamide, etc. In particular, styrene/maleic anhydride resins are widely used for surface sizing. Cellulose non-reactive sizes generally exhibit improved toner adhesion, little or no effect on coefficient of friction, no effect, or an improved effect on high speed converting, and no size reversion when compared to reactive sizes; however, they are less efficient at sizing than the reactive sizes.
As a result of all of the above, most papers at the present time are internally sized with alkenylsuccinic anhydride, alkyl ketene dimers, alkenyl ketene dimers or rosin sizes.
There is a need to provide surface sizing agents that overcome the problems enumerated above, because there are substantial advantages to be gained from surface sizing when compared to internal sizing. Among them are:
a) Efficiency. The surface size components are completely retained in the system; whereas in internal sizing a significant amount is lost in the white water. Moreover, in those applications where only a surface response is needed, surface sizing permits keeping the majority of the size at the surface, thus gaining the maximum response for a minimum amount of material.
b) Environmental. The high retention of surface size components minimizes environmental contamination.
c) Fiber bonding. Sizes applied at the surface are less likely to interfere with fiber-fiber bonding because the bonds have already been formed when the sizes are applied.
Now it has been found unexpectedly that treatment of paper with a combination of reactive and non-reactive sizes can produce paper with a unique balance of final properties that cannot be achieved by using either of the size types alone. The combination of cellulose reactive and cellulose non-reactive sizes provides paper that exhibits better water holdout than paper that is the same except that the sizing composition contains only cellulose non-reactive size. The combination size also provides paper that performs better in ink jet printing than does paper that is the same except that the size composition contains only cellulose reactive or only cellulose non-reactive size. Furthermore, the paper exhibits better toner adhesion, higher coefficient of friction and a lower coefficient of friction bandwidth than does paper that is the same except that the size composition contains only cellulose reactive size. The paper is also capable of performing effectively in tests that measure its convertibility on state-of-the-art converting equipment and its performance on high speed end-use machinery.
U.S. Pat. No. 5,498,648 discloses paper size mixtures which are prepared by mixing an aqueous suspension of a digested cationic starch with a finely divided, aqueous polymer dispersion which is the paper size and emulsifying C.sub.14 -C.sub.22 -alkyldiketene in this mixture at not less than 70.degree. C. It is taught in the patent that the size mixtures can be used for both engine (internal) and surface sizing. However, use as a surface size is mentioned only as a possibility; all of the examples and discussion pertain to internal sizing.